Chapter 16, Chemistry: A Molecular Approach
pH beyond the equivalence point
-log[H3O+]
What is the pH of a buffer solution when the concentrations of both buffer components (the weak acid and its conjugate base) are equal?
The pH = pKa of the buffer system
What happens to the pH when the buffer contains more of the weak acid than the conjugate base?
The pH becomes more acidic (drops). Can be calculated with the HH equation
What happens to the pH when the buffer contains more of the conjugate base than the weak acid?
The pH becomes more basic (pH rises). Can be calculated with the HH equation
Suppose that a buffer contains an equal amount of a weak acid and its conjugate base. What happens to the relative amounts of the weak acid and conjugate base when a small amount of strong base is added?
the strong base reacts with the weak acid fo the buffer system, converting it to a conjugate base of the buffer system. Thus, the weak acid concentration decreases, the conjugate base concentration increases, and the pH rises slightly at most
Suppose that a buffer contains equal amount of a weak acid and its conjugate base. What happens to the relative amounts of the weak acid and conjugate base when a small amount of strong acid is added to the buffer?
When the amount of acid = amount of base, the pH = pKa. When a small amount of strong acid is added, it reacts with the conjugate base of the buffer system, converting it to the weak acid of the buffer system. Then the weak acid concentration increases, the conjugate base concentration decreases, and the pH drops slightly at most.
What is a buffer? How does a buffer work? How does it neutralize added acid? Added base?
A buffer is a chemical system that resists pH changes. The buffer works by neutralizing an added acid or base. Most buffers contain significant amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid). When additional base is added to a buffer, the weak acid reacts with a base, neutralizing it (generating more of a buffer system conjugate base). When additional acid is added to a buffer, the conjugate base reacts with the acid, neutralizing it (generating more of the buffer system conjugate acid). In this way, a buffer can maintain a nearly constant pH
Describe acid-base titration. What is the equivalence point?
At the equivalence point--the point in the titration when the number of moles of base is stoichiometrically equal to the number of moles of acid--the titration is complete.
pH before the equivalence point (strong acid with a strong base)
Before the equivalence point, H3O+ is in excess. Calculate [H3O+] by subtracting the number of moles of added OH- from the initial number of moles of H3O+ and dividing by the total volume. Then convert to pH using -log[H3O+]
What is the common ion effect?
Occurs when a solution contains two substances (HC2H3O2 and NaC2H3O2) that share a common ion (C2H3O2-). The presence of the C2H3O2-(aq) ion causes the acid to ionize even less than it normally would, resulting in a less acidic solution (higher pH0. This effect is an example of Le Chatelier's principle shifting an equilibrium because of the addition, or removal, of a common ion from the solution
What factors influence the effectiveness of a buffer? What are the characteristics of an effective buffer?
Relative amount s of the acid and conjugate base (the closer they are to each other, the more effective the buffer). and the absolute concentrations of the acid and conjugate base (the higher the absolute concentrations, the more effective the buffer)
What is the Henderson-Hasselbalch equation and why is it useful?
The Henderson-Hasselbalch equation (pH = pKa + log (base/acid)) allows easy calculations of the pH of a buffer solution from the initial concentrations of the buffer components as long as the "x is small" approximation is valid.
How do you use the Henderson-Hasselbalch equation to calculate the pH of a buffer containing a base and its conjugate acid?
The pKa is the -log (equilibrium constant of the acid dissociation reaction where the weak acid component and water are the reactions and the conjugate base and H3O+ are products). At the end, confirm that "x is small" approximation is valid by calculating the [H3O+] concentration from the pH. Because H3O+ is formed by ionization of the acid, the calculated H3O+ has to be less than 0.05 (or 5%) of the initial concentration for the "x is small" approximation to be valid
What is the effective pH range of a buffer (relative to the pKa of the weak acid component)?
The relative concentrations of acid and conjugate base should not differ by more than a factor of 10 for a buffer to be reasonably effective. Using the HH equation, this means that the pH should be within one pH unit of the weak acid's pKa.
The pH at the equivalence point of the titration of a strong acid with a strong base is 7.0. However, the pH at the equivalence point of a weak acid with a strong base is above 7.0. Explain.
The titration of a weak acid by a strong base always has a basic equivalence point because at the basic equivalence point, all of the acid has been converted into its conjugate base, resulting in a weakly basic solution
The volume required to reach the equivalence point of an acid-base titration depends on the volume and the concentration of the acid or base to be titrated and on the concentration of the acid or base used to do the titration. It does not, however, depend on whether or not the acid or base being titrated is strong or weak. Explain.
The volume required to get to the equivalence point is only dependent on the concentration and volume of acid or base to be titrated and the acid or base used to do the titration because the equivalence point is dependent on the stoichiometry of the balanced reaction of the acid and base. The stoichiometry only considers the number of moles involved, not the strength of the reactants involved.
initial pH (strong acid with a strong base)
concentration of the strong acid
pH at the equivalence point
neither reactant is in excess and the pH = 7.0